Sprint now says it will cut in half the price of most Verizon, AT&T and T-Mobile plans, though some unlimited plans are excluded. New customers can sign up for the offer starting Friday through Jan. 7. The offer will be good until 2018.

For example, Verizon customers paying $80 for 12GB of data will pay $40 for 12GB if they switch to Sprint, and save 50 percent on the access fee as well.

The offer is aimed at promoting Sprint's new LTE Plus network which is available in 77 major markets.

Shares in Sprint Corp. ended down 41 cents, or 9.3 percent at $3.99. The stock is down about 4 percent for 2015.

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